As with all software, there comes a time when the software is no longer supported. There are MANY who complain about this in the Microsoft ecosystem which I find sad since 10 years is a pretty long time. For those that have old 2008 R2 boxes still in production and Windows 7, your time has come.
What does this mean to you as a user? It is not that the software will stop working. New features and upgrades have been gone for 5 years, so it is not that either. No, it is all about security and vulnerabilities. With the end of extended support all those machines that are unsupported will no longer receive (i.e. they are no longer being created) security updates. That means when a vulnerability is exposed - and there will be - the systems will not be patched. There are SCADA-type systems that could care less since they are off-net but I would bet you know of at least one server or PC that is no longer supported that is still on the network.
Reach out if assistance is needed and, in some cases, Microsoft has resources to assist. Microsoft was also kind enough to provide the following table (thank you Greg Taylor from the Exchange team) to help those plan (or react as the case may be).